in Analgesic Tablets Robert D. Braun University of Southwestern Louisiana, Lafayette, LA 70504 Acetylsalicylic acid, ASA, can he analyzed in several ways. Fluorometric and chromatographic procedures'4 have been published for use as student experiments in the analytical chemistry lahoratory. An attractive alternative to using an instrumental method is a titrimetric procedure. Titrimetric methods eenerallv vield more accniate and more nrecise results than instrigental methods. The classical m e t h o d b f analysis of ASA involves a back titration. The ASA is dissolved in a few milliliters of ethanol, and an excess of aclueous sodium hvdroxide solution is added. The sodium hydroxide reacts kith the acidic group on the ASA and hydrolyzes the ASA t o salicylate. The excess sodium hydroxide is hack titrated with 0.1 M hydrochloric acid to a phenolphthalein endpoint. ASA cannot he directly titrated with sodium hydroxide in aqueous solution because it is not sufficiently soluhle in water. A titrimetric procedure is described in the uresent oaDer . . which has been class tested in a sophomore-level analytical lahoratory. The procedure has been found to yield results that are superior to those obtained using the fluorometric and chromatographic methods. The procedure can he used in undergraduate analytical courses as well as in other selected lahoratory courses. The solvent for the titration is methanol. Consequently the experiment serves as an example of a titrimetric method in a nonaqueous amphiprotic solvent. Methanol is a good choice as aiolvent fo;anuudergraduate experiment because
it is relatively inexpensive, it requires no purification prior to use, and ASA is soluble in it. ASA is not soluble in water. The dielectric constant and autodissociation constant of methanol at 25°C are 32.63 and 2.0 X 10-17. res~ectivelv. That compares to values of 78.54 and 1.0 X 10-l4 for water. The titrant is a solution of sodium methoxide. In our undergraduate lahoratory, the students individually prepared sodium methoxide solutions by adding sodium to methanol in the hood. Although no accidents have occurred here, the dissolution is exothermic, and, in some schools, the instructor might choose to prepare the reagent. The use of sodium methoxide emphasizes t o the students that the strongest base or acid is dependent upon the solvent. The time required for a student to complete the entire experiment varied between 2 and 3 h.
1 Braun. Rooert D., and Wallers, Fred H.. "Applications of Chemlcal Analysis." McGraw-Hill. New York. 1982. Haddad. Paul. Hutchins. Stephen, ana Tuffy. Michael. J. C ~ E M . Eouc., 60, 165 (1983). Kagel, R. A., and Farwell, S. 0.. J. CHEM. EDUC., 60, 163 (1983). Beaver. Rodnev W., Bunch. John E., and Jones. Louis A.. J. CHEM. Eouc., 60, 1000 (1983). Kalthoff.I. M. and Stenger. V. A., "Volumetric Analysis." vol. II, 2nd ed., Interscience. New York, 1947, p. 233.
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Volume 62
Number 9
September 1985
811
Experimental Apparatus Titrations were performed w ~ t ha 50-mL buret. The titration vessels were 250-mL Erlenmeyer flasks. A total of seven flasks are required if the flasks are not cleaned between uses. Both 10- and 25-mL pipets are required for dilutions. Volumetric flasks that contain 100, 200 and 500 mL are used in the experiment. An analytical balance, a 1-L beaker, and a 50-mL graduated cylinder complete the list of required equipment. If the titration is performed potentiometrically, a pH meter, glass electrode, and Ag/ AgCl (sat'd KC1 in methanol) reference electrode are required as described in the Discussion. Chemicals Anhydrous methanol from any manufacturer is adequate as the solvent. The water in the solvent does not need to he removed prior to use. Metallic sodium (stored under oil) is used toprepare the sodium methoxide solution. It should he handled with care. If the students prepare the sodium methoxide solution themselves, they should he carefully supervised and use safety goggles. Reagent-grade ASA is used as the primary standard. The chemical used in our laboratory was obtained from the Aldrich Chemical Company. The chemical indicator used in the study was hromeresol purple (W. A. Taylor Co.). The samples were commercial, analgesic tablets such as extra-strength Excedrin tablets, Norwich aspirin tablets, Albertson's buffered aspirin tahlets, and Mack aspirin tablets. Any other commercial analgesic tablets which contain ASA also could he used. Procedure The followingprocedure is that given to the students. The steps are numbered for clarity. The last two steps of the procedure list the calculations that are required to obtain the percentage (wlw) of ASA in the analgesic tablet. The procedure is designed to prepare 0.01 M ASA solutions. At those concentrations, hlank corrections are not required. 11 PrrpareaO.1 A4sodium methuxidesolution by carefully nddingabout U.7 g of mrtallir svdium tonbot~t2% mLuf methanol in a I-L beaker. 2) After the sodium has dissolved, add 50 mL of the 0.1 M solution to a 500-mL volumetric flask. Fill the flask to the mark with methanol. The resulting solution should be about 0.01 M in sodium methoxide. 3) Weigh 0.45 g of reagent-grade ASA to the nearest 0.1 mg. Place the ASA in a 100-mLvolumetric flask and add about 50 mL of methanol. Place a cap on the flask and shake to dissolve the ASA. After the ASA has dissolved, dilute the solution in the flask to the mark with methanol. 4) Use a 10-mLpipet to add 10 mL of the ASA solution that was prepared in step (3) to eaeh of three 250-mL Erlenmeyer flasks. Add about 15 mL of methanol to each flask with a graduated cylinder. 5) Fill a 50-mL buret with the 0.01 M sodium methoxide solution that was prepared in step (2). Be sure to flush the air bubble from below the stopcock in the buret. Record the volume of solution in the buret. 6 ) Add 3 or 4 drops of bromcresol purple indicator solution to each of the three Erlenmeyer flasks that contain the ASA. Titrate each solution to the endpoint with the 0.01 Msodium methoxide solution. Record each endpoint volume to the nearest 0.01 mL. The color change of the indicator is from yellow to blue, hut the solution will appear to he green at the endpoint. Take the endpoint to correspond to the disappearance of the yellow. Sometimes it is easier to locate the endpaint if a f l a k which contains the ASA and indicator solution, but which has not been titrated, is held adjacent to the flask to which titrant is added. 7) Weieh the samnle . tahlet to the nearest 0.1 me. Record the sample number, i i i c has one. Dirsolvr the tablet inahout 100 ml. uf mcrhnnol in a 250-mL Erlenmeyer flask. If the enrw rabler doer nor dissolw, filter rhe solution to remove insdublr romponents. Somr non-AS.4 romponmts uf analgesic tablrr~are induhle in mrthanol If filrmrion is nrrwliary. rar~l'ullsrinse the walk oi rhe flask with rhrac 10-ml. portions ofmethanol, and pour each washing through the funnel and into the filtered solution of the sample. 8) Carefully pour the solution of the sample into a 200-mL ~
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Journal of Chemical Education
volumetric flask. Rinse the walls of the vessel in which the sample solution was contained prior to addition to the volumetric flask with three successive 10-mL portions of methanol, and add each portion to the volumetric flask. Dilute the solution in the flask to the mark with methanol. 9) Use a 25-mL pipet to deliver 25 mL of the solution in the 200mL volumetric flask into eaeh of three 250-mL Erlenmeyer flasks. Add 3 or 4 drops of brameresol purple toeach solution and titrate with the 0.01 M sodium methoxide solution. Record the endpoint of eaeh titration to the nearest 0.01 mL. 10) Use the mass of the weighed ASA, the various dilution factors, and the endpoint volumes of the three titrations performed in step ( 6 ) to calculate three values of the molarity of the sodium methoxide solution. The concentration should be calculated to four significant figures. Determine the mean and the standard deviation of the concentrations. 11) Use the mean sodium methoxide concentration from step (lo), the mass of the weighed analgesic tablet, the dilution factors used in preparing the titrated solution, the three endpoint volumes obtained in step (9), and the molecular weight (180.15) of ASA to determine three values of the weieht .. ner. cmrage of ASA in the analgasir tablet. Determine thq.mean and rhe standard drviation of the rrrulLs. The sodium merhox& reacts in a 1:I molar ratic, with ASA. Discussion Some analgesic tahlets t h a t contain ASA also contain one or more other constituents. T h e most common of t h e other components are acetaminophen, aluminum hydroxide, caffeine, and magnesium hydroxide. Acetaminophen is a n analgesic, caffeine is a stimulant, and aluminum hydroxide and magnesium hydroxide are buffering agents. None of those components interferes with the analisis. Apparently t h e phenolic proton on acetaminophen is not sufficientlv acidic react with t h e titrant. In step (1) of t h e procedure the instructions call for the addition of 0.7 g of sodium to methanol. T h e mass is in excess of t h a t theoretically required t o prepare a solution of t h e desired ronccntratiun il&ause the metal invariably has an oxidized layer as well as an adhering layer of oil which contribute tu the mass. T h r a d d i t i m should he done slowly and pieceaise to prevent overhmting. Other vhemical indicators that ran tre used in place of hn)mrresol purple are trromrhymol hlue and phenol red. Hromthymol hlue chance3 from yellow (acid, co lrlue (basic) and phenol red changes from-light yellow t o red. T h e bromthymol blue looks green a t the endpoint and the phenol red looks orange. T h e endpoint also can he located potentiometrically with a glass indicator electrode. T h e electrode must he soaked in methanol for a t least 24 h prior t o t h e titration in order t o obtain a suitable response. T h e suerested reference elec,... trude is a silwr silver chloride electrode dipped i n t u a metha n d i r solution that is saturated in silver chloride and notassium chloride. T h e silverlsilver chloride electrode can be prepared a s described elsewhere.' Potentiometric titrations caibe performed a t ASA concentrations t h a t are 1X 10-'M or higher. A hlank correction should he applied a t concentraM. T h e potential falls about 35 tions below about 1 X mV in t h e vicinity of the end point for titrations of 1 X M ASA with a 0.01 M sodium methoxide. A t higher ASA concentrations the decrease in potential in t h e vicinity of the endpoint is ereater. For the titration of 0.1 M A S A with 0.1 M sidiurn methoxide t h e decrease is about 130 mV.Each potentiometric titration twicallv. reauires between 30 and . 45 min for completion. ~ h time k required t o complete the experiment using t h e ~ o t e n t i o m e t r i ~c r o c e d u r eis about 4 h. 'I'hestudentsused both the titrimeiricand the fluurometric' prncedures to analyze identical analgesic tahlets. T h e fluoiometric analysis typically required about 3 h t o complete a n d yielded results t h a t were less accurate and less precise than those ohtained using the titrimetric procedure. Some students did have trouble recognizing the endpoint. Those students generally benefited from performing more than t h e triplicate titrations called for in t h e procedure. ~~~
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